Lifetimes of electrodes with a relatively small amount of the active material in the coating (e.g. less than 7.5 g/m.sup.2) rapidly decrease with an increase in current density. In general, an early failure of an electrode is attributed to two major factors, loss of the active coating and dissolution, or in case of the film-forming metals, passivation of the substrate. Sometimes these occur simultaneously and the electrode at the end of its lifetime may show some active material left in the coating but the substrate passivated. A common solution to the problem of loss of the active component in the coating and passivation of the substrate, in the art, is use of thicker coatings i.e. higher loadings of the active component. Thicker coatings produced by brushing onto the substrate several (e.g. ten-twenty) layers of the active coating proved beneficial for lifetimes of the electrodes with the same coating composition. Simplicity of the solution to the problem of electrode lifetimes made thicker coatings a popular and almost universal remedy. However, this simple approach is found effective only up to a point and under certain electrochemical conditions (e.g. relatively low current densities, less corrosive environments, etc.) In addition, an increase of the coating thickness means a significant increase in cost.
The problem of electrode lifetime is particularly important with oxygen evolving electrodes used as anodes in various industrially important electrochemical processes e.g. metal electrowinning, electroforming, electroflotation, and electrosynthesis. In these processes, electrodes with platinum-group metal oxide coatings are used as oxygen evolving anodes. These platinum metal oxide anodes are found to operate very well under relatively difficult conditions imposed by these processes (e.g. current densities of up to 2-3 kA/m.sup.2 in aggressive electrolytes). However, to attain an acceptable performance, under these conditions, these electrodes must have relatively high platinum-group metal loadings (e.g more than 4.5-7 g/m.sup.2). Various tests with the known oxygen evolving anodes have shown, however, that while electrodes with platinum-group metal oxides operate with satisfaction under these conditions they fail rapidly if the operating current density is increased to 5 kA/m.sup.2 or more. The simple approach of a higher loading therefore meant only higher costs but not better service life. In recent years, the rapid development of high speed plating (electrogalvanizing) techniques has amplified the problem.
It has been known from U.S. Pat. No. 3,711,385 that the electrocatalytic coating of a platinum-group metal oxide could be made as thin as 0.054 micrometers. In practice, however, it has been found that to achieve any acceptable lifetime somewhat thicker coatings were necessary. Hence, usually ten to twenty thin coatings of a suitable paint solution are applied to the film-forming metal base and heated each time to give an electrocatalytic coating formed from the decomposed component of the paint containing about 5 to 20 grams by metal of the platinum-group metal oxide per square meter of the projected electrode surface.
Many attempts have been made to ecomonize on the precious metal content of these coatings, usually, by partly replacing the platinum-group metal oxide by a compatible non-precious metal oxide such as tin dioxide (see for example U.S. Pat. No. 3,776,834) or tin and antimony oxides (see for example U.S. Pat. No. 3,875,043).
Another electrode for oxygen-evolution is that described in GB No. 1 399 576, having a coating containing a mixed crystal of tantalum oxide and iridium oxide. However, known electrodes of this type contain at least about 7.5 g/m.sup.2 of iridium so that despite their excellent performance in terms of over-voltage and lifetime, the high cost of iridium makes these electrodes less attractive.
The electrode proposed in GB No. 1 463 553 has a base which consts entirely or at its surface of an alloy of a film-forming metal and an activating metal for instance a platinum-group metal, whose surface is oxidized during use or is preactivated by an oxidizing treatment to form in the outer part of the alloy a surface oxide layer to a depth of 1 to 30 micrometers. Such alloys have shown promise for electrowinning but are quite difficult to prepare by sintering or in another manner and are quite expensive because of the quantity of platinum-group metal in the alloy. Also, the pre-activation methods are difficult to control to obtain an improvement in the electrode performance.
An electrode with a titanium substrate and an active platinum/iridium metal coating has been disclosed in GB No. 964 913. The electrode is produced by thermal decomposition of platinum and iridium compounds in a reducing atmosphere at 350.degree. C. By modifying this process it has been possible to produce coatings of platinum and iridium oxide.
An oxygen evolving anode made by coating a titanium substrate with iridium oxide or iridium/ruthenium oxide using a mixture of codedeposited titanium oxide or tin oxide and tantalum oxide or niobium oxide with platinum metal as the electrode underlayer has been disclosed in U.S. Pat. No. 4,481,097. The electrode active component includes 1.3 g/m.sup.2 of platinum metal in the underlayer and 3.0 g/m.sup.2 of iridium oxide in the toplayer. According to the document the electrode has maximum life time of 80 hours under accelerated lifetime tests performed in an aqueous solution with 150 g/l of H.sub.2 SO.sub.4 as an electrolyte at 80.degree. C. and current density of 25 kA/m.sup.2.
An electrode with a titanium substrate and an electrocatalyst which preferably comprises up to 0.5 g/m.sup.2 of iridium oxide and/or rhodium oxide per projected electrode surface has been disclosed in EP No. 0 046 447. According to the disclosure the electrocatalyst is formed as an integral surface film of an oxide or another compound of titanium metal which is grown from the substrate which incorporates iridium oxide and/or rhodium oxide as electrocatalyst. The electrode is produced using a method in which a solution of thermally decomposable compound of iridium and/or rhodium and an agent which attacks the metal of the substrate are applied to the titanium substrate and the coated structure then heated in air at 500.degree. C. A superior performance for the electrode disclosed over the previous oxygen evolving anodes was demonstrated for processes in which the electrode was used at current densities between 500 and 1000 A/m.sup.2. It could not be suspected that electrodes produced according to the principle disclosed in this teaching could prove to be useful and have an outstanding lifetime in processes operating at a high current density